(1) Field of the Invention
This invention relates to a plasma display panel used as a display device and the production method, and in particular to a plasma display panel suitable for a high-quality display.
(2) Description of the Prior Art
Recently, as expectations for high-quality and large-screen TVs such as high-vision TVs have increased, displays suitable for such TVs, such as CRT, Liquid Crystal Display (LCD), and Plasma Display Panel (PDP), have been developed.
CRTs have been widely used as TV displays and excel in resolution and picture quality. However, the depth and weight increase as the screen size increases. Therefore, CRTs are not suitable for large screens exceeding 40 inch in size. LCDs have high performance such as low power consumption and low driving voltage. However, producing a large LCD is technically difficult and the viewing angles of LCDs are limited.
On the other hand, it is possible to produce a large-screen PDP with a short depth, and 40-inch PDP products have already been developed.
PDPs are divided into two types: Direct Current type (DC type) and Alternating Current type (AC type). Currently, PDPs are mainly AC type since these are suitable for large screens.
FIG. 1 shows a perspective view of a conventional AC PDP.
In FIG. 1, the element 101 is a front glass substrate (front panel) and the element 105 is a back glass substrate (back panel). These substrates are made of soda lime glass.
The front glass substrate 101 with display electrodes 102 thereon is covered with a dielectric glass layer 103, which functions as a capacitor, and with a magnesium oxide (MgO) dielectric protecting layer 104.
The back glass substrate 105 with address electrodes 106 thereon is covered with a dielectric glass layer 107. Partition walls 108 are attached onto the dielectric glass layer 107 and fluorescent substance layers 109 are inserted between the partition walls 108. Discharge gas is injected into discharge spaces 110 sealed by the front glass substrate 101, the back glass substrate 105, and the partition walls 108.
Silver electrodes or Cr--Cu--Cr electrodes are used as the display electrodes 102 and the address electrodes 106. The silver electrodes can be easily formed with the printing method.
As the demand for high-quality displays has increased, PDPs with minute cell structures have been desired.
For instance, in conventional 40-inch TV screens of National Television System Committee (NTSC) standard, the number of cells is 640.times.480, cell pitch 0.43 mm.times.1.29 mm, and area of one cell about 0.55 mm.sup.2. On the contrary, in 42-inch high-vision TVs, the number of cells is 1920.times.1125, cell pitch 0.15 mm.times.0.45 mm, and area of one cell 0.072 mm.sup.2.
In a minute cell structure, the distance between discharge electrodes (display electrodes) becomes short and the discharge space small. As a result, it is necessary to make the dielectric layer thinner than conventional one to maintain as large capacitance of the dielectric layer as conventional one.
However, glass used for the dielectric glass layer, such as lead oxide glass or bismuth oxide glass, has inferior wettability with metal materials used for electrodes. Therefore, it is difficult to coat these electrodes with a thin and even dielectric glass layer and these electrodes have a problem concerning withstand voltage. Since there are prominent projections and depressions on the surface of silver electrodes, in comparison with Cr--Cu--Cr electrodes, it is particularly difficult to coat the silver electrodes with a thin and even dielectric layer and the withstand voltage problem is notable.
With regard to the above problems, Japanese Laid-Open Patent Application No. 62-194225 discloses a technique to form a thin and even dielectric layer by forming an inter-layer between electrodes and a dielectric layer. The inter-layer is formed by applying SiO.sub.2 and Al.sub.2 O.sub.3 on a substrate with an electrode before a dielectric glass layer is formed.
This disclosure describes specific methods for forming the inter-layer. According to the disclosure, the inter-layer is formed by applying silica solution onto the surface to have 500-10000 A thickness with the spin-coat method or the dipping method, and by baking the layer. The Japanese Application also discloses another method in which a material of the inter-layer is applied onto the surface by the EB (electron beam) evaporation method or the sputtering method.
Although the above techniques improve the withstand voltage to a certain extent, further improvement in the withstand voltage is desirable.
When a PDP having the structure in FIG. 1 is produced, electrodes, dielectric layers, and partition walls are formed in that order on a glass substrate made of soda lime glass. In each step of the above formation, a material is applied onto the surface and is then baked with some method.
For instance, a dielectric layer 103 is formed by applying lead-oxide-based glass material onto the surface to have a thickness ranging from 20 .mu.m to 30 .mu.m and by baking the applied glass material (see Japanese Laid-Open Patent Application No. 7-105855), where the lead-oxide-based material includes lead oxide (PbO), boron oxide (B.sub.2 O.sub.3), silicon dioxide (SiO.sub.2), zinc oxide (ZnO), and aluminum oxide (Al.sub.2 O.sub.3), and has relatively low melting point in a range of 500 to 600.degree. C. and a thermal expansion coefficient in a range of 80.times.10.sup.-7 /.degree. C. to 83.times.10.sup.-7 /.degree. C.
The partition walls are also formed by applying glass materials with the screen printing method and baking the applied glass materials.
When a thin glass substrate is used, electrodes, partition walls, dielectric layers, and fluorescent substance layers may crack or the glass substrate may warp or shrink when they are baked at heating temperature of 500-600.degree. C. Thermal expansion coefficients of their materials are different so that, when the materials are heated, partition walls, dielectric layers, and the like are distorted and cracks are easily caused in dielectric layers and partition walls. The cracks caused in the dielectric layers reduce the withstand voltage.
In view of the above problems, it is necessary to use a glass substrate with a certain thickness, which becomes a factor for increasing the weight of a large-screen PDP.
For instance, for a 42-inch TV, the size of the glass substrate is about 97 cm.times.57 cm, and, to prevent warping and shrinkage, the thickness is set to about 2.6-2.8 mm.
The specific gravity of the glass is 2.49 g/cm.sup.3 so that, if the substrate is 2.7 mm in thickness, the total weight of the front and back glasses is about 7.4 Kg and the weight of the panel to which circuits are attached exceeds 10 Kg (see Display And Imaging, Vol.14, PP96-98, 1996, for instance).
Regarding these problems, a glass substrate having a relatively high distortion point has been developed (PD-200 made by Asahi Glass co. has the distortion point of 570.degree. C., for instance). By using this glass substrate, it is possible to reduce deformations, such as warping and shrinkage, of the glass substrate in the heat treatment (see Display And Imaging, Vol.14, PP99-100, 1996, for instance).
The specific gravity of this PD-200 glass is, however, 2.77 g/cm.sup.3 and this value is greater than 2.49 g/cm.sup.3, which is the specific gravity of soda lime glass. The Young's modulus of PD-200 glass is greater than that of soda lime glass and the thermal expansion coefficient of PD-200 is 84.times.10.sup.-7 /.degree. C., similar to that of soda lime glass. As a result, using such a glass having a high distortion point does not significantly reduce the panel weight (see Electric Display Forum 97, P6-8, Apr. 16-18, 1997, for instance).